Common mode current is one of the very popular topics on social media, but one does not see much quantitative discussion, mostly just a lot of hand waving without supporting measurement data.
There are a a number of commercial instruments instruments purporting to measure RF feed line common mode current (Icm), but many are specious frauds.
There are quite a few articles on the ‘net describing DIY ‘balance measurement’ techniques / instruments and again, many are specious frauds.
There are a very small number of commercial instruments and DIY designs that would appear to be soundly based, many of them are ‘uncalibrated’ in that they do not give absolute current readings, and even relative readings may be impacted by non-linear response.
So, what are we trying to measure?
Bear in mind that Icm is usually a standing wave, and ‘measurement’ means a survey at several points along the feed line.
In my experience, for HF, Icm with a 100W transmitter feeding a reasonably efficient antenna system is likely to result in Icm greater than about 10mA, and more that 100mA is poor… so the measurement range of interest is really between 10mA and 100mA.
Scale this to the transmit power level that will be used for measurement:
- for 1000W the measurement range of interest is really between 30mA and 300mA; and
- for 10W the measurement range of interest is really between 3mA and 30mA.
An example Icm meter
Above is the meter scale from an Icm meter with absolute calibration described at Measuring common mode current . It has two current ranges, the 1000mA FSD is close to linear from 100mA to 1000mA, the 100mA FSD is quite non-linear. The article describes how the calibration and meter scale were developed.
Unless Icm is very poor, this meter will be used on the 100mA range, even at high power. It is not really sensitive enough for QRP power levels where you would want to read down to a few mA or less.
A more sensitive Icm measurement
A series of articles at https://owenduffy.net/blog/?s=current+probe describes an RF current probe that could be used with:
- a broadband RF power meter;
- a (broadband) detector;
- Spectrum Analyser (RFPM2 – current probe – #4 – TinySA Ultra).
This probe can achieve calibrated sensitivity down to -75dBA or 0.2mA, or even less. The frequency selectivity of a Spectrum Analyser becomes necessary at very low Icm to separate most external sources.
Conclusions
- Popular techniques for ‘balance measurement’ are often unsound, betraying a lack of understanding of the concepts of transmission lines and common mode current.
- Measurement of Icm is an important basis for assessing antenna systems, common mode problems and solutions.
- Absolute measurement is much more useful that relative measurement.
- For common station configurations, good measurements in the range 10-100mA are needed.
- Instrument sensitivity and calibration linearity are key challenges.